If filtration devices are utilized for long periods of time, filtration media (filtration sand) within filtration tanks of the filtration devices become clogged. This prevents the performance of efficient filtration, and deteriorates the quality of filtered liquids, such as water. For this reason, clogging is eliminated by removing contaminants, which are attached to the filtration media. In order to perform operations efficiently, it is desirable that the process of removing the contaminants from the filtration media, that is, a cleansing process, be performed in a short amount of time with few steps. It is also desirable that the cleansing process be performed without taking up additional space. For these reasons, it is considered to provide filtration sand cleansing devices (filtration media cleansing mechanisms) within filtration tanks. The provision of cleansing mechanisms within filtration tanks would enable efficient cleansing of filtration media in a short amount of time, without removing the filtration media from within the filtration tanks.
There are known filtration devices, as disclosed in Japanese Patent No. 31491 and Japanese Unexamined Utility Model Publication No. 63 (1988)-98704, which have been developed in view of these points. The filtration device disclosed in Japanese Patent No. 31491 comprises a central pipe (cleansing tank), which is suspended by a frame (support portion). The lower end of the central pipe opens within a filtration chamber (filtration tank). A propeller is provided within the interior of the central pipe, at its lower end. A pipe having a jet expulsion opening is provided above the propeller, the jet expulsion opening being slightly above the upper edge of the central pipe. The pipe having the jet expulsion opening is linked to the propeller, and rotates therewith, to spray cleansing fluid in the horizontal direction by use of centrifugal force. During filtration, water containing contaminants (hereinafter, referred to as “raw water”) is supplied from above, and is filtered by passing through filtration sand, which is provided on an apertured false bottom (filter bed). During cleansing, the propeller is rotated, causing filtration sand to be suctioned into the central pipe via the lower opening thereof. The filtration sand is elevated through the central pipe, then discharged in the horizontal direction by the cleansing fluid being sprayed from the jet expulsion opening. The filtration sand is cleansed by contaminants being separated therefrom at this time.
The filtration device disclosed in Japanese Unexamined Utility Model Publication No. 63-98704 comprises an elevating pipe (cleansing tank) erected therein, and a spiral water elevator provided within the elevating pipe. During filtration, water is discharged by a water dispersing tube, which is within filtration sand. Processed water, which has been filtered by passing through the filtration sand from below, is discharged above the filtration sand. During cleansing of the filtration sand, the spiral water elevator rotates and elevates the filtration sand, which has trapped contaminants, from the lower portion of the spiral water elevator. The contaminants are separated from the filtration sand by use of centrifugal force. The filtration sand is expelled through a filtration sand expulsion opening, which is provided in the upper portion of the elevating pipe. The filtration sand then returns to the interior of the filtration tank.
As a further example of a known filtration device, there is that which is disclosed in Japanese Unexamined Patent Publication No. 8 (1996)-215509. This filtration device is configured similarly to that disclosed in Japanese Unexamined Utility Model Publication No. 63-98704, in that raw water is filtered by moving upward through a filtration tank. A cylinder that contains a screw conveyor is suspended from the upper portion of the filtration tank. Filtration sand is cleansed during upward conveyance thereof from the lower end to the upper end of the cylinder by the screw conveyor. The filtration sand, which has been conveyed upward, is further agitated within a separation chamber provided at the upper portion of the cylinder, to remove contaminants therefrom. The cleansed filtration sand is then returned to the upper surface of the filtration media layer. In this filtration device, raw water is caused to flow upward from the bottom thereof during cleansing. That is, filtration is performed continuously, without interruption.
Conventionally, in filtration devices of the type in which raw water is supplied from above and permeates through to the lower portions thereof, configurations are known, in which layers of large diameter gravel is provided on filter beds, and finer sand is provided on top of the gravel layer.
There are cases in which cleansing of filtration media is performed daily, for example, prior to the start of business, or after close of business. In the case that filtration devices are operated 24 hours a day, the degree of clogging by contaminants may be detected with sensors to automatically perform cleansing as necessary. Alternatively, there are cases in which cleansing is automatically performed based on a timer, that operates every predetermined time interval before clogging occurs.
Conventionally, there are also cases in which great numbers of particles are compressed to a degree that spaces remain between the particles, to serve as strainers.
In the filtration device disclosed in Japanese Patent No. 31491, filtration media is provided directly on the apertured false floor. This causes problems in that filtration sand passes through the apertures of the false floor, flow downward, and become mixed into the filtered water. In the case that the water to be filtered contains concentrated contaminants, the apertures may become clogged. During cleansing, contaminants are removed by rotation of the propeller and cleansing water is backwashed toward the filtration sand from the apertured false floor, to separate and expel the contaminants attached to the filtration sand. However, the total area of the plurality of apertures, which are provided in the apertured false bottom, is limited, that is, the open area ratio is low. Therefore, cleansing water cannot be backwashed uniformly onto the filtration sand on the apertured false floor. As a result, the efficiency during rinsing of contaminants from the filtration sand is low, and the cleansing operation requires time.
The filtration devices disclosed in Japanese Unexamined Utility Model Publication No. 63-98704 and Japanese Unexamined Patent Publication No. 8-215509 are of the upward flow type. That is, normal filtration is performed by supplying raw water from a lower supply opening, then causing it to move upward. Upward flow type filtration devices are generally provided for the purpose of processing a great amount of raw water. Therefore, the flow rate is high, causing filtration media toward the upper portion to float. This increases the spaces between the particles of filtration sand, which causes a problem in that contaminants within the water are not sufficiently trapped.
Particularly, the filtration device disclosed in Japanese Unexamined Patent Publication No. 8-215509 does not cease filtration even during cleansing. Therefore, fine light sand is pressed upward, and coarse sand is retained at the lower portion thereof. If filtration media in this state is elevated by the spiral water elevator (screw conveyor), the coarse lower layer of filtration sand is elevated, then discharged onto the sand layer. The coarse sand sinks to the bottom and the fine light sand remains at the upper portion. Accordingly, a problem arises in that only the coarse sand is cleansed, while the fine sand remains contaminated. For this reason, filtration devices that perform continuous filtration have not been put to practical use. In addition, because cleansing water is not backwashed upward from the filter bed, the expulsion of separated contaminants cannot be performed efficiently, which causes the cleansing operation to take time.
In the case that filtration media, in which a layer of fine sand is provided on top of a layer of large diameter gravel, the fine sand is prevented from sinking to the lower portion by the large diameter gravel. There are other advantages, such as apertures in the filter bed being less likely to become clogged, and the flow of liquid being uniform, by being dispersed by the large diameter gravel layer. However, the following problem arises when cleansing filtration media provided in this manner.
In the case that cleansing is performed only by backwash cleansing, in which cleansing water is caused to be sprayed from the filter bed, the cleansing water is sprayed upward through paths, which are not blocked by contaminants, in the layers of filtration media. Therefore, filtration sand in the peripheries of the paths move, causing unevenness in the gravel layer, that is, unevenness at the surface of the gravel layer to be generated. Then, when filtration is performed following cleansing, raw water that flows through the filtration media is not uniformly dispersed, due to paths therefor being skewed by the unevenness. Thereby, the filtration efficiency deteriorates, or the filtration effects do not stabilize. In the case that a screw conveyor is utilized to cleanse the filtration sand above the gravel layer, there is a possibility that the rotation of the screw conveyor will affect the upper portion of the gravel layer, to cause unevenness therein.
In addition, in the case that sand layers are provided on top of gravel layers, three to four layers of gravel and sand of gradually decreasing particle diameters are provided. This is to ensure that fine sand does not drop into the layer of large gravel. In this case, each layer must be of approximately the same thickness. Therefore, the entire layer thickness becomes great, and as a result, the height of the filtration tank also increases. Accordingly, there is a possibility that restrictions will arise in the installation location of the filtration device, if installed indoors. Further, maintenance and management, such as cleansing, of a plurality of different types of filtration media is difficult.
Further, strainers of the type in which great numbers of particles are compressed are likely to become clogged depending on the contaminants, and it is difficult to remove the contaminants that cause the clogging.